In many situations, it is desired to be able to provide an accurate and complete measurement of one or more periodically modulated signals using a receiver or measurement device whose bandwidth is less than the bandwidth of the periodically modulated signal.
FIGS. 1A-C illustrate an example of this situation.
FIG. 1A shows an example frequency spectrum 110 of a periodically modulated input signal provided to a device under test (e.g., an amplifier) and an example frequency spectrum 120 of an output signal of the device under test. Here is illustrated an example where the bandwidth of the example frequency spectrum 120 of an output signal of the device under test is somewhat greater than the bandwidth of the spectrum 110 of a periodically modulated input signal, for example due to spectral regrowth in the device under test (e.g., an amplifier operating at least somewhat into compression). Of course it is understood that in general the bandwidth of the frequency spectrum of a periodically modulated input signal provided to a device under test and the bandwidth of the frequency spectrum of an output signal of the device under test may be the same as each other or different from each other.
FIG. 1B shows the example frequency spectrum 112 of the periodically modulated input signal, downconverted to baseband with a first mixing frequency (e.g., LO1), and the example frequency spectrum 122 of the output signal of the device under test, also downconverted to baseband with the first mixing frequency LO1, together with the spectrum of a filter 130 having the limited bandwidth RBW of a receiver which is used to measure and characterize the periodically modulated input signal and the output signal of the device under test. Here it is assumed the bandwidth of the downconverted output signal of the device under test is SBW>RBW.
FIG. 1C shows the portion 114 of the spectrum of the periodically modulated input signal and the portion 124 of the spectrum of the output signal of the device under test which are actually able to be measured and characterized by the receiver with the limited bandwidth RBW. As denoted in FIG. 1B, a portion 113 of the spectrum of the periodically modulated input signal, and a portion 123 of the spectrum of the output signal of the device under test, are not measured by the receiver because of the limited bandwidth RBW.
Thus it would be desirable to provide a convenient and reliable method and system to measure and characterize a periodically modulated signal, and an output signal of a device under test (DUT) produced in response to the periodically modulated signal, using a receiver whose bandwidth is less than the bandwidth of the periodically modulated signal itself and/or the bandwidth of the output signal. It would further be desirable to provide such a system and method which can provide accurate measurements of phase sensitive characteristics, such as the error-vector-magnitude (EVM), for a DUT.